The plasminogen system is composed of the zymogen, plasminogen, the active serine protease, plasmin, and the activators, inhibitors and modulators of plasmin activity. This system plays an indispensable physiological role by mediating the lysis of fibrin deposits and, thereby, controlling vascular patency and tissue integrity. In addition to fibrin, plasmin cleaves a variety of matrix proteins, activates numerous metalloproteinases and regulates the activities of multiple growth factors and cytokines. These additional substrates of plasmin suggest that the plasminogen system may facilitate cell migration. The development of Pig1 mice has provided a direct means to assess the functions of plasminogen in cell migration, and support is evolving for this role. The capacity of plasminogen to influence cell migration depends upon its interaction with plasminogen receptors, a heterogeneous set of cellular proteins with C-terminal lysines that interact with the lysine binding sites (LBS) in the kringle domains of plasmin(ogen). Cell migration is an elicited response; and, therefore, modulation of plasminogen binding to cells provides a potentially important regulatory mechanism. Marked up- and down-regulation of plasminogen binding to cells have been demonstrated, but the mechanisms underlying these changes remain obscure. In Aim 1, a unifying membrane remodeling hypothesis to explain the regulation of plasminogen binding to cells will be tested. This hypothesis postulates that a variety of cellular responses, ranging from cell adhesion to apoptosis to oncogenic transformation, modulates the cell surface to expose proteins with C-terminal lysines that serve as plasminogen receptors. This hypothesis will be tested not only in vitro, but also by examining changes in plasminogen binding and plasminogen receptor expression in vivo. In Aim 2, the presumed contribution of the LBS within the plasminogen kringles to the fibrinolytic and cell migratory functions of plasminogen will be tested in vivo. These studies will utilize plasminogen derivatives lacking specific kringles, either prepared by biochemical approaches or expressed as transgenes in P1g-/- mice to determine how the LBS contribute to the biological functions of plasminogen in vivo. Biological modifiers of LBS activity, molecules that interfere with their function, could exert profound effects on the plasminogen system. Two such biological modifiers are apo(a), which is composed of multiple kringles, including ones with LBS activity, and plasma carboxypeptidaseB, TAFI, which removes the C-terminal lysines from degrading fibrin and cell surfaces. In Aim 3, the influence of these two biological modifiers on the functions of plasminogen will be analyzed in vivo using mice expressing apo(a) or deficient in TAR. Taken together, these studies seek to bring insights into the physiological and pathophysiological roles of the plasminogen system and its regulation.